you have to crawl underneath, and on the front/driver's side of the differential there is a plug. and you need a 3/8" drive rachet to stick into this plug then remove it. no socket, just the square drive of the rachet or extension.

What I did when I had to add a second bottle, is I syphoned into a styrofoam cup what looked like MORE than the bottle of fluid your adding, then pour in the bottle of modifier, (awkward angle yes- can use a hose on the tip) then fill up the modifier bottle with the fluid you drained, and top it up.